# 5.5. The Computation of Total Field Differences

Whenever possible, observatories should measure the scalar field intensity S (Section 6.1.3) in addition to the vector components of the magnetic field. The total field component F (Section 6.1.3) can be calculated from the vector components of the field using simple mathematical relationships. F and S thus provide two independent values of the total field intensity from two different instruments at different locations within the observatory. Calculating the difference between these two values (F - S) is a powerful tool for quality control of the magnetic time series data.

Where both instruments are stable and functioning correctly the value (F - S) is expected to be a constant (within the noise envelope of the instrumentation) and by accounting for the difference in the magnetic field between the two locations where F and S are measured the (F-S) value can be reduced to close to zero. Any variations in the value of (F - S) which are beyond the expected noise envelope of the two instruments can indicate contamination or instability in either the vector or scalar instrument. (F - S) is often the only measure of the vector variometer baseline stability during the periods between absolute observations.

The (F - S) value can be computed from adjusted, quasi-definitive or definitive 1 minute or 1-second data and used to monitor data quality and instrument stability as part of routine daily quality checks.

The value of this technique as a quality control tool is diminished when the calculated total field (F) is dominated by one component of the magnetic field. In this case changes in F can be relative insensitive to the component which contributes least to the value of F.